Direct control switching systems, also known as step-by-step systems were developed many years ago to replace the previous manual telephone switches. In recent times there has been a conversion to common control switching systems which make use of digital techniques. The common control systems have been installed in many of the large exchanges and in new exchanges. However, there are still many local telephone exchanges which still operate with direct control switching systems. Many of these exchanges have such a large investment in capital equipment that it is not economical to purchase current technology digital switches. But as a result of the divestiture of the Bell System and the need for additional telephone service features, there exist a need for substantial technical improvements for the existing telephone exchanges having step-by-step equipment.
In the more modern, common control switching system, dialing is completed before the switching takes place. When the dialing has been completed through the common control equipment, the digits are decoded and then a connection is set up through a switching matrix to connect the parties. The common control equipment is then available for handling another call. Since the common control equipment is used only for a few seconds for each call, it is possible for a very large number of calls to be set up through the switching equipment.
The common control switching architecture has evolved into modern electronic analog and digital switching systems. The older direct control switching technology has been limited to individual circuit improvements and external circuit add-ons for improving and adding particular functions.
In the direct control switching system, the call progresses through the office as each digit is dialed until the desired number is reached. One unit of equipment translates each digit and connects the call to the next stage of selection for the next digit. The equipment which is used for the selection remains held in the transmission path for the duration of the call. Upon completion of the call, all of the equipment used for the call is released.
The existing step-by-step switching systems, direct control architecture, which is in use throughout the telephone industry, provides basic telephone service by the use of electromechanically controlled switching devices known as line finders, selectors and connectors. Auxiliary trunk circuits are connected to provide special services such as coin box, long distance, directory service, custom calling features, etc.
Each of the three types of step-by-step switches consists of three basic parts, an electromechanical relay assembly, the stepping mechanism, and a disconnect mechanism.
The electromechanical relay assembly provides the intelligence and control for a particular type switch. This assembly provides the following features:
1. Recognizes seizure and disconnect (supervisory function).
2. Drives the vertical and rotary stepping magnets in response to the digit dialed.
3. Tests for busy or idle trunks, lines or telephone numbers in the bank multiples.
4. Connects signaling and status tones such as dial tones, busy, overflow and audible ring to the called line.
5. Connects ringing current to the called line.
6. Provides talking battery and ground to the talking path.
7. Detects hang-up and activates the switch release mechanism.
The stepping mechanism includes magnets, ratchets, springs and other devices. This mechanism operates under the direction of the relay assembly for causing the shaft and associated brush or wiper assemblies to connect to the desired bank terminal in response to the digit which was dialed.
The disconnect mechanism is much like the stepping mechanism, but causes the switch to return to its normal idle position when directed by the relay assembly at the time of subscriber disconnect.
The bank multiple terminates subscriber lines (line finder) telephone numbers (connector), or trunks (selectors) to succeeding switching stages in the dial train. There are usually three terminals associated with each bank location. Two of the terminals make up the signaling and talking circuit with the third providing supervisory or control functions. The brushes or wipers of the switch shaft provide the electrical connection from the preceding circuit to the next switching stage of the switch train.
A primary limitation of the conventional step-by-step telephone switch is its lack of capability for identifying a calling party, for call routing and auxiliary service provision. In past years, due to the existence of a unitary telephone company, such capability was not provided in the smaller step-by-step offices. While add-on equipment for provision of number identification for billing purposes (ANI) is available, internal line indentification within the line group on every call has not been available. However, in light of the new regulatory environment with the breakup of the Bell System, and customer demand for custom calling features in all offices, the need for line identification exists.
In summary, due to the continued wide-spread use of step-by-step telephone switches in many telephone exchanges, the changing business nature of the telephone system and the requirement that local exchanges be more cognizant and exercise more control of local operations, there exists a need for apparatus for updating the capabilities of step-by-step telephone switches without total replacement of exchange equipment.